1. Field of the Invention
The present invention relates in general with both magnetometers and magnetic microscopes and in particular with a magneto-optical microscope magnetometer (MOMM) capable of a simultaneous measurement of local hysteresis loops and local activation magnetic moments of submicrometer-scale local areas (about 0.3xc3x970.3 xcexcm).
2. Description of the Prior Art
Generally, magnetic materials continue to be widely used in traditional industry as permanent magnets, transformers, motors, etc. Recently, such magnetic materials have been more extensively studied for use as advanced materials for information storage medias, magnetic sensors, etc. In the magnetic information technology, information is stored in the form of magnetic domains and the magnetization reversal process of the domains under applied magnetic fields is basically involved. It is generally considered that the detailed domain structure and domain reversal dynamics depend on local microscopic magnetic properties. The local magnetic properties are, in general, spatially inhomogeneous due to structural and/or chemical imperfections. Therefore, to achieve high-performance of the magnetic information technology, it is important to characterize and monitor the local magnetic properties with a high spatial resolution.
Thus, much effort has been devoted to developing magnetic microscopes capable of measuring magnetic properties of a local area. As such magnetic microscopes, there have been utilized (i) a magneto-optical microscope adopted from optical microscopy, (ii) a magnetic force microscope and near-field scanning optical microscope adopted from scanning microscopy and (iii) a scanning electron microscope, Lorentz transmission electron microscope and low-energy electron microscope adopted from electron microscopy. However, the above-mentioned magnetic microscopes, except for the magneto-optical microscope, cannot measure the dynamic characteristics of magnetic materials under applying a magnetic field, for instance, a hysteresis loop nor an activation magnetic moment, due to limitations imposed on the impossibility of applying a magnetic field and/or slow data acquisition time.
On the other hand, there [have] has been developed several measurement techniques for probing the dynamic characteristics, for instance, the hysteresis loop and the activation magnetic moment, of magnetic materials. First, the hysteresis loop is a curve indicating a magnetized state of a magnetic material depending on the strength of an external magnetic field. This curve continues to be one of the most representative data used for the measurement of magnetic properties of a magnetic material, from which curve can be obtained magnetic information, for instance, a coercivity, magnetization reversal mechanism, magnetic domain formation, etc. Second, the activation magnetic moment signifies a basic magnetic moment of a magnetic material acting as a single particle when the magnetization of the magnetic material reverses by applying a magnetic field that is externally applied to the magnetic material. The activation magnetic moment is the most basic physical quantity describing a dynamic characteristic of a magnetic material, where the activation magnetic moment can be obtained from the dependence on an external magnetic field and magnetization reversal dynamics of the magnetic material.
Up to the present, the vibrating sample magnetometer (or magneto-optical magnetometer) has generally been used to measure the hysteresis loop and activation magnetic moment. However, even though such conventional magnetometers can microscopically measure the hysteresis loop and activation magnetic moment over the entire area of a magnetic material, they cannot measure them for a submicrometer-scale local area of the magnetic material.
To sum up, conventional magnetometers can microscopically measure a hysteresis loop and activation magnetic moment over the entire area of a magnetic material but cannot measure them for a submicrometer-scale local area of the magnetic material, whereas conventional magnetic microscopes can observe magnetic properties of the submicrometer-scale local area of the magnetic material, but cannot measure the hysteresis loop and activation magnetic moment.
Therefore, the present invention has been motivated to overcome the above problems, and it is an object of the present invention to provide a magneto-optical microscope magnetometer which is capable of measuring a hysteresis loop and activation magnetic moment of a submicrometer-scale local area by performing both functions of a conventional magneto-optical microscope and conventional magneto-optical magnetometer.
In accordance with the present invention, the above objects can be accomplished by a magneto-optical microscope magnetometer that comprises an electromagnet unit for applying a magnetic field to a magnetic material; a polarizing optical microscope for magnifying and visualizing a magnetized state of the magnetic material via the magneto-optical effect; a camera system for detecting the image visualized by the polarizing optical microscope; a data analysis system for obtaining a hysteresis loop or an activation magnetic moment from the time-resolved images grabbed by the camera system; and a magnetic field control system to remotely control the electromagnet for applying the magnetic field into the magnetic material.
Preferably, the polarizing optical microscope may include a light source; a polarizer for linearly polarizing a light beam from the light source; a beam splitter for reflecting the light beam from the polarizer to an objective lens; an objective lens for focusing the light beam from the beam splitter onto the magnetic material and then, collimating the light beam reflected from the magnetic material to an analyzer; an analyzer for converting the polarized light beam from the objective lens into the image intensity by linearly polarizing it; and a camera lens for focusing the image from the analyzer to the camera system.
Further, the camera system may include an image intensifier for amplifying the image intensity from the polarizing optical microscope; a charge coupled device (CCD) camera for detecting the image amplified by the image intensifier; and an image grabber for grabbing the image detected by the camera into a digital signal.
Further, the electromagnet unit may include an electromagnet for generating the magnetic field; and a power supply for the electromagnet.
More preferably, the data analysis system may obtain the polar Kerr hysteresis loop from the images grabbed by the camera system. The data analysis algorithm is developed based on equation 1, below, which describes the relation between a Kerr angle and a Kerr intensity, where the Kerr angle is a rotational angle of the polarized light during reflection at the magnetic material via the magnetooptical Kerr effect, while the Kerr intensity is the light intensity detected by the camera system due to the magnetooptical Kerr effect. The polar Kerr hysteresis loop can be obtained from the Kerr intensity variation with respect to the strength of the applied external magnetic field, using equation 2, below, which is converted from the equation 1.
I(H)=I0+C sin2 (xcex8(H) +xcex1H+xcex94xcex8)xe2x80x83xe2x80x83[Equation 1]
                              [                                    θ              ⁡                              (                H                )                                                    θ              M                                ]                =                              [                                          Δ                ⁢                                  xe2x80x83                                ⁢                θ                                            θ                M                                      ]                    +                                    [                              α                                  θ                  M                                            ]                        ⁡                          [                                                                                                                  I                        ⁡                                                  (                          H                          )                                                                    -                                              I                        0                                                                                    C                                              a                        2                                                                                            -                H                            ]                                                          [Equation  2]            
where I is the Kerr intensity measured at a unit CCD pixel of the camera system, I0 is an intensity offset for the given CCD pixel, C is a proportional constant of the Kerr rotation angle, xcex1 is a Faraday constant at the objective lens, xcex94xcex8 is an angle between the polarizer and analyzer, xcex8M is a maximum Kerr rotation angle when the magnetic material is saturated, and H is a strength of the magnetic field applied to the magnetic material.
In addition, the data analysis system may obtain the activation magnetic moment from the time-resolved image detected by the camera system; the switching time of the magnetic material under an applied magnetic field is measured from the temporal variation of the Kerr intensity measured by the camera system and then, the activation magnetic moment is determined from the field dependence of the switching time using equation 3:
xcfx84=xcfx840exp((EBxe2x88x92mAH)/kBT)xe2x80x83xe2x80x83[Equation 3]
where xcfx84 is a magnetization switching time depending on the magnetic filed H applied to the magnetic material, xcfx840 is a characteristic switching time, mA is the activation magnetic moment, kB is a Boltzmann constant and T is a temperature.
In a feature of the present invention, there is provided a magneto-optical microscope magnetometer capable of performing the functions of a conventional magneto-optical microscope and a conventional magneto-optical magnetometer by using an individual CCD pixel in the conventional magneto-optical micrometer as a photo detector in the conventional magneto-optical magnetometer. Thus, the magneto-optical microscope magnetometer can measure a hysteresis loop as well as an activation magnetic moment in a submicrometer-scale local. Further, the magneto-optical microscope magnetometer can simultaneously measure hysteresis loops and activation magnetic moments for all the CCD pixels and thus, it can generate the 2-dimensional distribution of the coercivity and activation magnetic moment for the entire magnetic material.